Is "Water Dependence" Really a Thing?—The Science of "Just Right" Hydration to Survive the Scorching Heat

1 Introduction: Water Protects Life, Yet Sometimes Threatens It

Japan's summers are becoming increasingly hotter due to climate change, with the number of heatstroke alert days issued by the Ministry of the Environment increasing by about 1.5 times compared to the early 2020s. While hydration is essential in the heat, we often hear news about people collapsing from drinking too much water.Proper hydration requires the right "amount," "timing," and "composition" to be safe—this is the core of this article.

2 The True Nature of the Term "Water Dependence"

2-1 The Difference Between the Coined Term "Dependence" and Medical Terminology

The term "water dependence" used in everyday language is not an academic term. The medically related concepts are as follows.

2-2 The Increasing Background of Excessive Water Drinking Domestically and Abroad

• Social Media Challenges: Videos of "chugging 1 gallon (about 3.8 L) of water" went viral, with reports of emergency hospitalizations.

• "Dehydration Fear": In elderly care facilities, staff give excessive instructions to "drink before sweating."

• Diet Myths: The misconception that "drinking water boosts metabolism" leads young women to regularly consume 8 L a day.
  
  

3 Fluid Dynamics in the Body on Hot Days

3-1 The Mechanism of Sweating

The human body releases water and electrolytes from sweat glands to lower body temperature.A typical adult walking outdoors at 4 km/h for 30 minutes in midsummer will sweat about 0.5 L, which contains 40–80 mmol of sodium.

3-2 Simple Estimation Formula for Sweat Volume

1. Measure body weight naked before and after exercise

2. Weight loss (kg) × 1000 = Sweat volume (mL)

3. Add the amount of beverage replenished (mL) and convert to per 30 minutes
   
   

4 Risk of Heatstroke and Dehydration

Dehydration is said to decrease physical performance by 1%, reduce concentration by 2%, and sharply increase the risk of heatstroke by 3%.

5 Process of Developing Water Intoxication

1. Ingesting a large amount of low-electrolyte water in a short time

2. Decrease in plasma osmolality

3. Water moves into cells causing cerebral edema

4. Headache, Vomiting→Convulsions→Consciousness Disturbance→Respiratory Arrest
   
   

Drinking more than 0.8 L per kg of body weight per houris reported to be high risk. Cases of drinking over 10 L in 24 hours also exist.

6 Guidelines for Safe Fluid Intake

6-1 Baseline (Normal Days)

According to the Japanese Dietary Reference Intakes (2025 Edition),the recommended total daily fluid intake is set at 35 mL per kg of body weight.For a 60 kg person, this is about 2.1 L. Of this,approximately 1.2 L should be from beverages,with the remainder covered by food and metabolic water.

6-2 Additional Amounts in Hot Environments

7 Tips for Electrolyte Supplementation

• Commercial Sports Drinks: Na 40–50 mg/100 mL. Ideal for intense exercise or heavy sweating.

• Oral Rehydration Solutions (ORS): Na 80–110 mg/100 mL. For suspected heatstroke, diarrhea, or vomiting.

• Homemade Recipe: 1 L water + 3 g salt + 20 g sugar + lemon juice.
  
  

Potassium and magnesium can be balanced by supplementing with foods like bananas, nuts, and spinach.

8 Checkpoints by Age and Condition

Elderly

• Less aware of thirst.

• Visualize room temperature and humidity (thermo-hygrometer),encourage 100 mL every hourwith verbal prompts.
  
  
  

Children

• High body surface area ratio leads to rapid dehydration.

• Provide water or diluted sports drinks every 30 minutesat a guideline of body weight×5 mL.

Heart, Kidney Diseases, Pregnant Women

• Prioritize doctor's instructions, restrict fluids if necessary.

• If there is a history of hyponatremia,dilute ORSfor use.
  
  

9 Drinking and Management Techniques

1. Frequent small amounts: 200 mL every 15 minutes.

2. Temperature: 5–15 ℃ is optimal for absorption speed. Room temperature for those with weak stomachs.

3. Diuretic beverages: Caffeine limit is 400 mg per day. Alcohol promotes dehydration.

4. Urine color check: Pale lemon color is ideal. Colorless and transparent is a sign of overhydration.

5. App utilization: Record weight, intake, and outdoor temperature, and set warning alerts.
   
   

10 Self-check & Emergency Response

Six self-check items

If three or more items apply, review fluid intake and consult a medical institution.

Emergency Response

• Dizziness/Headache:Stop intake and consume a small amount of sodium-containing beverage

• Vomiting/Consciousness disturbance:Call 119, measure body temperature and vitals

• Do not force drinks during transport
  
  

11 What can be done at workplaces, schools, and communities

• Corporate heatstroke guidelines: Mandate "15-minute break at WBGT 28 ℃" based on OSHA standards.

• School club activities: Coaches measure the weight loss rate of athletes before and after practice and manage it within 2%.

• Municipalities: Open stations and libraries as cooling shelters and install water coolers.
  
  

12 Future Outlook—"Water Literacy" in the Era of Global Warming

The IPCC Sixth Assessment Report predicts that the average annual temperature in Tokyo will rise by 3°C by 2100. The number of heatstroke fatalities is expected to increase to 1.8 times the current level by the 2040s. It is urgent to formally introduce water literacy education into the health and physical education curriculum in elementary and junior high schools and to promote continuous learning in workplaces and communities.

13 Summary

1. "Drinking too little" and "drinking too much" can both be fatal.

2. The guideline is body weight × 35 mL + sweat loss, and the timing is "frequently in small amounts."

3. Don't forget electrolytes. Do not drink large amounts of water alone.

4. Monitor urine color, body weight, and temperature, and develop a habit of controlling the appropriate amount yourself.

5. If you feel any abnormalities, seek medical attention early.
   
   

By following these five principles, you can safely lead an active life even in the midsummer of Japan.

14 List of Reference Articles

1. National Federation of Workers and Consumers Insurance Cooperatives
   "What is Water Intoxication and Polydipsia? Know the Causes and Measures to Be Careful!"

2. Asahi Shimbun Digital
   "Scary Water Intoxication: Be Cautious of Heatstroke, But Also Beware of Drinking Too Much Water" (2024-08-10)

3. Ministry of Health, Labour and Welfare
   "Let's Drink Water for Health" Promotion Leaflet (2025 Edition)

4. Japanese Association for Acute Medicine
   "Heatstroke Medical Guidelines 2024"

5. Ministry of the Environment
   "Recommendations for Basic Measures to Prevent Heatstroke in the Workplace 2025"

6. Japan Sports Association (currently: Japan Sport Association)
   "Guidelines for Fluid and Salt Intake During Sports Activities" Revised 3rd Edition

7. American College of Sports Medicine
   Exercise-Associated Hyponatremia Consensus Statement, 2023